The present invention relates to solid state microwave amplifiers. In particular, the present invention relates to solid-state millimeter-wave amplifiers, i.e. amplifiers which can operate at frequencies above 30 GHz. In particular, the present invention relates to solid state two-port amplifiers.
It has long been known that a number of possible mechanisms could provide a diode with gain, i.e. a two-port amplifier Device structures which provide this include tunnel diodes, Gunn diodes, and various kinds of avalanche transit-time devices and hybrids thereof, such as IMPATTS, TRAPATTs, baritts, tunetts, etc.
To get reasonably high power out of these possible solid state mechanisms, e.g. to get a power of at least several watts at frequencies above 30 GHz, it is desirable to use a distributed structure, that is a structure in which the gain medium of a diode is distributed over a substantial area, and a laterally propagating wave is amplified due to gain properties of the diode.
However, a great difficulty in achieving amplification rather than oscillation with such structures has been self-isolation. That is, a distributed active medium which is capable of amplifying a small input signal will presumably also be capable of amplifying small reflected signals from its own output. At higher micro-wave frequencies, it is exceedingly difficult to achieve output transitions and connections which do not cause at least some reflection, and it has therefore been very difficult to operate a distributed diode device in a configuration which reliably will not include resonances.
Thus it is an object of the present invention to provide a distributed diode with gain which is self-isolating.
It is a further of the present invention to provide a distributed diode with gain which is self-isolating and can provide substantial output power at microwave frequencies at 30 GHz.
It is a further object of the present invention to provide a distributed diode with gain which provides unidirectional amplification.
It is a further object of the present invention to provide a distributed diode with gain which provides unidirectional amplification at higher microwave frequencies.
It is a further object of the present invention to provide a distributed diode with gain which provides unidirectional amplification at frequencies above 30 GHz at power levels above 1 watt.
It is also desirable to be able to fabricate device structures meeting the above criteria monolithically. That is, it is now possible to fabricate devices for small-signal amplification and signal processing at microwave frequencies on chip. Using MESFETs or HEMT (FET using wide-band gap material to provide a two dimensional electron gas under the gate), it is even possible to reliably fabricate such devices for operation at frequencies above 30 GHz. However, large signal amplification remains a problem. In particular, even where a subsequent power amplifier stage can be provided, the wave length dimensions at millimeter-wave frequencies are so small that mechanical difficulties in fabrication mean that interconnections between discrete packages may themselves act as antennas. Thus, a substantial fraction of the power output from a chip may be dissipated as radiation. Moreover, radiated power from other sources may also be coupled into the interconnections inadvertently. Finally, there are many applications, such as millimeter phased array radar and tactical infantry communications, where even the modest amounts of power which are thermally safe in integated circuit packaging would be exceedingly useful as an RF output. Thus it is an object of the present invention to provide an integrated circuit capable of amplifying microwave signals above 30 GHz to power levels more than 100 milliwatts.
It is a further of the present invention to provide an integrated circuit capable of amplifying microwave signals above 30 GHz to power levels of 1 watt.
The present invention satisfies these and other objectives by providing a distributed microwave diode which has a tapered active region. Preferably an IMPATT diode is used. The diode is operated in the power-saturated region. That is, normal IMPATT operation takes place in the vertical direction, within a conventional IMPATT structure (e.g. two heavily doped horizontal contact layers, vertically separated by a lightly doped drift region). An RF signal propagating along this diode (which may be considered as a parallel-plate transmission line) will couple to the vertical IMPATT currents, so that the RF signal experiences gain in the horizontal direction. This will increase until the horizontally propagating RF signal is in a power saturating mode, i.e. the gain of the device. In the present invention, the diode is made long enough that, after the propagating RF signal is already voltaged saturated, it continues to propagate through a diode section wherein the width of the diode is constantly increasing. That is, since the vertical thickness of the diode active region remains the same, the saturated RF voltage remains the same, but, since the width of the diode is now increased, the RF current increases. Thus, additional power gain is provided in the tapered portion of the diode. Unidirectionality is provided in the large-signal case, because of the power saturation effects. That is, as a reflective signal (a backward wave perturbation) propagates backward along the tapered diode region, it encounters region of steadily decreasing gain. That is, as the width of the diode active region narrows (in this direction) the gain of the diode decreases. The backwards-propagating wave will thus be amplified for some distance, but will eventually encounter a diode region where the backward-propagating wave is enough to reduce the gain of the diode locally below the local loss of a diode. (In the power-saturated condition which prevails at every point of the tapered region of the diode, the gain would be almost equal to the loss anyway.) Thus, the diode wil become losy at this point, and the amplitude of the backward-propagating wave will be reduced. This will also reduce the amplitude of the forward propagating wave, but the forward-propagating wave will typically be restored to the power-saturation point on the gain curve as it propagates forward into the higher-gain portions for the device. This will not occur to the backward-propagating wave, which will be damped.
According to the present invention there is provided:
1. A microwave amplifier comprising:
A semiconductor diode having top and bottom contact and an active region inbetween, said semiconductor diode being forlonged to define a transmission line;
Said a transmission line being tapered, so that said semiconductor diode active region is narrower at a first end thereof than at a second end thereof.